Corrected entry: When the jewel is converted into the phone booth about 10 minutes in, there is a slight inconsistency between shots. The phone booth start to take shape from the crystal. The next shot with Rufus in his glasses, if you look closely, the phone booth is fully formed in the reflection, as you can see the word "Phone" at the top of the booth. In the next shot, the phone booth still isn't complete.
Corrected entry: In the scene when everyone is in the mall Beethoven is in the music store. The sales clerk turned on the drum machine on the keyboard and Beethoven reacted. This should not have happened since Beethoven was completely deaf - the movie had already established that Beethoven was deaf at that time. When Bill and Ted "landed" the telephone booth to get Beethoven he was playing the piano in front of a gathering of people. The phone booth made a loud noise that everyone listening to Beethoven play reacted to, however, Beethoven continued to play without a reaction - thereby exposing his deafness.
Corrected entry: In the scene where Ted falls down a spiral staircase, the stairs should spiral anti-clockwise, not clockwise. This is because in medieval times, the castle's defenders would have an easier time defending in this direction as almost all of them would have been right-handed.